Typically, telephone data transmission is in a continuous mode system with conventional differential phase shift keying (DPSK) or frequency shift keying (FSK) modulation over a 2-wire cable pair. Other techniques for achieving full duplex bidirectional transmission which have advantages over a continuous mode system are also known. Disadvantages of a continuous mode system in full duplex bidirectional transmission result from the separation of both streams of data. Typically, frequency separation or algebraic cancellation is used to separate the data. However, frequency separation may be complicated to implement and is both bandlimited and distance limited. Algebraic cancelling is not exact for all applications and may also create a severe echo problem from various harmonics and transmission line reflections. Therefore, a promising alternative to continuous mode operation is a time compression multiplexed (TCM) system which is also known as a burst mode or ping-pong system. The burst mode system divides the two directions of transmission by separating them in time so that at any particular point in time, transmission is unidirectional. The penalty which results from using the burst mode system is a substantially higher frequency of transmission since data must be bursted at least at twice the frequency of the data entering the system to insure adequate time separation. Whenever DPSK or FSK modulation is used in a burst mode system, a very wide spectral content results. A further disadvantage results for FSK modulation because D.C. balance is usually not maintained for each burst of data and an extra bit is required to maintain D.C. balance. When a digital burst mode transmission is not D.C. balanced bit by bit, inter-symbol interference results and may destroy the meaning of the transmitted data. Whenever phase modulation is used, D.C. balance is maintained, but harmonic energy may radiate into other communication channels.